This invention relates to a hydraulic weighing apparatus and method. In particular, the invention relates to an apparatus and method for dynamic weighing, in which the weight to be measured is moved by a hydraulic cylinder, a rotary actuator, or the like, and in particular, to such a method and apparatus for use in weighing commercial and/or domestic waste
1. Background of the Invention
Waste or refuse is produced by both domestic households and commercial organisations. The service of waste removal is commonly carried out by municipal councils, particularly in the case of domestic households. In some arenas, waste is also collected by commercial organisations often acting on behalf of municipal councils. Most domestic households, pay a fixed rate for disposal of refuse which is independent of the amount of refuse the household produces. However, as councils become increasingly concerned about the environment, the cost of landfill, and the need to encourage people to recycle more household waste, attention is now being paid to reducing the amount of refuse households provide and to encourage them to recycle as much of their waste as possible. Further, governments are putting pressure oil councils to reduce the amount of waste that goes to landfill. This can only be achieved by separating recyclable material and biodegradable materials that previously went to landfill, from waste. Thus, many municipal councils are now considering introducing incentive schemes where residents pay for the refuse collection service and have weight limits.
Most industrial weighing systems operate using a load cell which is mounted on a structural member and the deformation of the member is used to calculate the weight of the object. However, static weighing is much too slow for use in a waste collection environment, since such technology, and other weighing technology requires a set period of time to perform the weighing function. This would increase the time required to complete the waste collection service and would be unacceptable in terms of efficiency. Thus, any practical weighing system for use in waste management must be able to weigh dynamically whilst the waste is actually being loaded onto a refuse collection vehicle.
Although dynamic weighing systems have been proposed, most of these systems are unreliable and inaccurate often because they fail to compensate for variables in the weighing process.
It is an object of the present invention to provide an apparatus and method which alleviates the problems of the prior art and enables dynamic weighing of waste bins and the like within acceptable degrees of accuracy.
2. Summary of the Invention
Thus according to the present invention, there is provided a hydraulic lifting system/apparatus comprising a lifting arm, frame or the like adapted to carry a load whose weight is to be measured, the arm or frame being moved by a hydraulic actuator means such as a hydraulically operated piston and cylinder arrangement, or a rotary actuator or the like, operable to cause the arm to move due to the flow of hydraulic fluid into or out of the hydraulic actuator means, including a first pressure sensing means for measuring the pressure of the hydraulic fluid in the system, and characterised by means for determining changes in the flow characteristics of the hydraulic fluid and means for calculating the weight of the load based on the pressure measurements made by the first pressure sensing means while compensating for changes in flow characteristics as measured by the means for determining changes in the flow characteristics
In a preferred embodiment, the means for determining changes in the flow characteristics include an orifice disposed in the hydraulic line. The first pressure sensing means is disposed on one side of the orifice. A second pressure sensing means is disposed on the opposite side of the orifice. The measurements from the first and second pressure can be used to calculate a pressure differential across the orifice. The kinematic viscosity of the hydraulic fluid is related to the pressure differential either side of the orifice plate. Knowledge of the kinematic viscosity of the hydraulic fluid can be used to compensate for changes in hydraulic fluid flow characteristics (which are primarily brought about by an increase in temperature) which would otherwise affect the accuracy of the weight measurement. Variations of as much as 300% in the measured value of the weight could occur between weight measurements made when the hydraulic fluid is cold and measurements made when the hydraulic fluid has reached its normal working temperature, if no account is taken of changes in flow characteristics.
Typically, each pressure sensing means will make a series of pressure measurements each defining a sequence of pressure measurements which can be used to produce an average pressure measurement for each pressure sensing means.
Another potential source of changes in flow rate, arise from the use of valves to operate lifting members. If the valve is not fully opened by the operator, variations in flow rate each time the arm is lifted, will occur.
In an alternative embodiment, instead of the orifice, the apparatus includes a temperature sensor. As the hydraulic fluid warms up its kinematic viscosity decreases and it becomes more free flowing. The temperature of the oil can be used to indirectly calculate changes in flow characteristics. However pressure difference measurement is preferable as it is more accurate and responsive than calculating theoretical changes due to temperature changes.
The apparatus may incorporate both the orifice and the temperature sensor.
In one embodiment, a hydraulic pump which pumps fluid to the hydraulic actuator to move the lifting arm operates at a constant volume flow rate during most of the movement of the arm, except for the periods when the arm stops and starts moving. The pressure measurements made by the first and second pressure sensing means are taken between two set points in most cases, while volume flow rate is theoretically constant and the arm is ideally travelling at a constant velocity, thus obviating the requirement for any accelerometer or other means for measuring the acceleration of the arm.
However, for hydraulic flows which provide non-uniform movement between the two sensors, an accelerometer could be used to detect that non-uniform movement and this information could be imported into the main calculation.
However, a constant flow rate pump is not essential and it is possible to calculate flow rate and variations in flow rate from measurements of the time it takes the lifting arm to move a set distance.
It is preferred that the apparatus includes a means for measuring the time it takes for the lifting device to move between the two set points. The time taken for the arm to move between the two points gives the flow rate. Corrections can be made to account for any variations in the normally constant velocity of the lifting arm.
The system of the present invention is particularly suitable for use on waste collection vehicles, however it could also be used on other hydraulic lifting apparatus such as forklifts. It will be appreciated that existing waste collection vehicles can be simply converted to the system of the present invention by for example, simply placing an orifice and pressure sensors into the vehicle""s hydraulic line plus other sensors including temperature sensors if desired, and link them to appropriate control and calculation means.
The system simply uses the existing hydraulics of the vehicle and it is not necessary to fit load cells or other weight sensing devices to the lifting mechanism to make weight measurements. Thus, the system of the present invention can be implemented relatively simply and without major mechanical modifications.
The system also preferably includes level measurement apparatus which is adapted to compensate for variations in the attitude of the base frame to which the lifting arm is attached which would affect the measured weight.
The invention also encompasses a method of dynamically weighing a load carried on a lifting member moved by a hydraulic actuator, the hydraulic actuator being operable to cause the lifting member to move due to the flow of hydraulic fluid into or out of the hydraulic actuator means, comprising the steps of, during the lifting of the weight by the lifting member;
measuring the pressure of the hydraulic fluid in the system using a first pressure sensor;
measuring variations in flow characteristics of the hydraulic fluid in the system;
calculating the weight of the load based on the pressure measured by the first pressure sensors, the calculating step including compensating for variations in flow characteristics of hydraulic fluid.
In one embodiment the step of measuring variations in the flow characteristics includes the step of;
measuring the pressure of the hydraulic fluid in the system using a second pressure sensor, wherein the first and second sensors are disposed either side of an orifice in the hydraulic line and calculating the pressure differential across the orifice.